Triazine UV absorbers are an important class of organic compounds which have a wide variety of applications. One of the most important areas of applications is to protect and stabilize organic materials such as plastics, polymers, coating materials, and photographic recording material against damage by light, heat, oxygen, or environmental forces. Other areas of applications include cosmetics, fibers, dyes, etc.
Triazine derived UV absorbers are a class of compounds that typically include at least one 2-oxyaryl substituent on the 1,3,5-triazine ring. Triazine based UV absorber compounds having aromatic substituents at the 2-, 4-, and 6-positions of the 1,3,5-triazine ring and having at least one of the aromatic rings substituted at the ortho position with a hydroxyl group or blocked hydroxyl group are generally preferred compounds. 
In general this class of triazine UV absorber compounds is well known in the art. Disclosures of a number of such trisaryl-1,3,5-triazines can be found in the following U.S. patents, all of which are incorporated by reference as fully set forth herein: U.S. Pat. Nos. 3,118,887; 3,242,175; 3,244,708; 3,249,608; 3,268,474; 3,423,360; 3,444,164; 3,843,371; 4,619,956; 4,740,542; 4,775,707; 4,826,978; 4,831,068; 4,962,142; 5,030,731; 5,059,647; 5,071,981; 5,084,570; 5,106,891; 5,185,445; 5,189,084; 5,198,498; 5,288,778; 5,298,067; 5,300,414; 5,323,868; 5,354,794; 5,364,749; 5,369,140; 5,410,048; 5,412,008; 5,420,008; 5,420,204; 5,461,151; 5,476,937; 5,478,935; 5,489,503; 5,543,518; 5,538,840; 5,545,836; 5,563,224; 5,575,958; 5,591,850; 5,597,854; 5,612,084; 5,637,706; 5,648,488; 5,672,704; 5,675,004; 5,681,955; 5,686,233; 5,705,643; 5,726,309; 5,726,310; 5,741,905; and 5,760,111.
A preferred class of trisaryltriazine UV absorbers (UVAs) are based on 2-(2,4-dihydroxyaryl)-4,6-bisaryl-1,3,5-triazines, i.e., compounds with two non-phenolic aromatic groups and one phenolic aromatic group advantageously derived from resorcinol. The 4-hydroxyl group of the parent compounds, 2-(2,4-dihydroxyaryl)-4,6-bisaryl-1,3,5-triazines, are generally functionalized to make 2-(2-hydroxy-4-alkoxyaryl)-4,6-bisaryl-1,3,5-triazine compounds for end use.
A number of commercial products exist in which the para-hydroxyl group of the phenolic ring is functionalized and the non-phenolic aromatic rings are either unsubstituted phenyl (e.g., Tinuvin® 1577) or m-xylyl (e.g. Cyasorb® UV-1164, Cyasorb® UV-1164L, Tinuvin® 400, and CGL-1545). These UV absorbers are preferred because they exhibit high inherent light stability and permanence compared to other classes of UV absorbers such as benzotriazole and benzophenone compounds.
There are several processes known in the literature for the preparation of triazine based UV absorbers. (See, H. Brunetti and C. E. Luethi, Helvetica Chimica Acta, 1972, 55, 1566-1595, S. Tanimoto et al., Senryo to Yakahin, 1995, 40(120), 325-339).
A majority of the approaches consist of three stages. The first stage, the synthesis of the key intermediate, 2-chloro-4,6-bisaryl-1,3,5-triazine, from commercially available materials can involve single or multi-step processes. Thereafter in the second stage, 2-chloro-4,6-bisaryl-1,3,5-triazine is subsequently arylated with 1,3-dihydroxybenzene (resorcinol) or a substituted 1,3-dihydroxybenzene in the presence of a Lewis acid to form the parent compound 2-(2,4-dihydroxyaryl)-4,6-bisaryl-1,3,5-triazine. The parent compound 2-(2,4-dihydroxyaryl)-4,6-bisaryl-1,3,5-triazine, as mentioned above, maybe further functionalized, e.g., alkylated, to make a final product 2-(2-hydroxy-4-alkoxyaryl)-4,6-bisaryl-1,3,5-triazine.
There have been several approaches reported in the literature on the synthesis of the key intermediate 2-chloro-4,6-bisaryl-1,3,5-triazine. Many of these approaches utilize cyanuric chloride, a readily available and inexpensive starting material. For example, cyanuric chloride is allowed to react with aromatics (ArH, such as m-xylene) in the presence of aluminum chloride (Friedel-Crafts reaction) to form 2-chloro-4,6-bisaryl-1,3,5-triazine, which is allowed to react in a subsequent step with resorcinol to form 2-(2,4-dihydroxyphenyl)-4,6-bisaryl-1,3,5-triazine (See, U.S. Pat. No. 3,244,708). There are several limitations to this process, viz., the reaction of cyanuric chloride with aromatics is not selective and leads to a mixture of mono-, bis-, and tris-arylated products including unreacted cyanuric chloride (See, Scheme 1). The desired product, 2-chloro-4,6-bisaryl-1,3,5-triazine, must be isolated by crystallization or other purification methods before further reaction. 
Another major drawback of the above mentioned process is that the reaction of cyanuric chloride with aromatics is not generally applicable to all aromatics. It is well known in the literature that the process provides a useful yield of the desired intermediate, 2-chloro-4,6-bisaryl-1,3,5-triazine, only when m-xylene is the aromatic reagent (GB 884802). With other aromatics, an inseparable mixture of mono-, bis-, and trisaryl products are formed with no selectivity for the desired 2-chloro-4,6-bisaryl-1,3,5-triazine (See, H. Brunetti and C. E. Luethi, Helvetica Chimica Acta, 1972, 55, 1575; and S. Tanimoto and M. Yamagata, Senryo to Takahin, 1995, 40(12), 325-339). U.S. Pat. No. 5,726,310 describes the synthesis of m-xylene based products. 2-chloro-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine is first synthesized and without isolation allowed to react with resorcinol in a one-pot, two-step process to produce 2-(2,4-dihydroxyphenyl)-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, which is subsequently purified by crystallization. A one pot process for preparing asymmetric tris-aryl-1,3,5-triazines from cyanuric chloride as well as from mono-aryl-dichloro triazines was earlier described in U.S. Pat. No. 3,268,474.
Several approaches were developed in an attempt to solve the above mentioned problems related to the formation of the key intermediate 2-chloro-4,6-bisaryl-1,3,5-triazine from cyanuric chloride. For example, cyanuric chloride is allowed to react with an aryl magnesium halide (Grignard reagent), to prepare 2-chloro-4,6-bisaryl-1,3,5-triazine (See, Ostrogovich, Chemiker-Zeitung, 1912, 78, 738; Von R. Hirt, H. Nidecker and R. Berchtold, Helvetica Chimica Acta, 1950, 33, 365; U.S. Pat. No. 4,092,466). This intermediate after isolation can be subsequently reacted in the second step with resorcinol to make a 2-(2,4-dihydroxyphenyl)-4,6-bisaryl-1,3,5-triazine (See, Scheme 2). This approach does not selectively synthesize 2-chloro-4,6-bisaryl-1,3,5-triazine; the mono- and tris-arylated products are formed in significant amounts (See, H. Brunetti and C. E. Luethi, Helvetica Chimica Acta, 1972, 55, 1575). Modifications with better results have been reported (See, U.S. Pat. No. 5,438,138). Additionally, the modified process is not suitable for industrial scale production and is not economically attractive. 
Alternate approaches were developed to solve the selectivity problem when synthesizing 2-chloro-4,6-bisaryl-1,3,5-triazine using either a Friedel-Crafts reaction or Grignard reagents, however, all solutions required additional synthetic steps. One approach, is outlined in Scheme 3. In the first step, cyanuric chloride is allowed to react with 1 equivalent of an aliphatic alcohol to make in high selectivity a monoalkoxy-bischlorotriazine. In the second step, monoalkoxy-bischlorotriazine was allowed to react with aromatics in the presence of aluminum chloride to prepare intermediates monoalkoxy/hydroxy-bisaryltriazines. These intermediates were then converted to 2-chloro-4,6-bisarayl-1,3,5-triazines in the third step by reaction with thionyl chloride or PCl5. In the fourth step, 2-chloro-4,6-bisaryl-1,3,5-triazines were allowed to react with resorcinol to synthesize 2-(2,4-dihydroxyphenyl)-4,6-bisaryl-1,3,5-triazines. In the above process, the desired product was formed with high selectivity. However, the two additional steps required made the process less attractive economically as an industrial process.
A similar approach is outlined in Scheme 4 (See, U.S. Pat. Nos. 5,106,972 and 5,084,570). The main difference is that cyanuric chloride was first allowed to react with 1 equivalent of alkanethiol, instead of an alcohol. As with the process summarized in Scheme 3, additional steps were required, making the process neither efficient nor economically feasible. 
Recent improvements are disclosed in European patent application 0,779,280 A1 and Japanese patent application 09-059263.
Other approaches do not utilize cyanuric chloride as a starting material. For example, the synthesis of 2-chloro-4,6-bisaryl-1,3,5-triazine as disclosed in EP 0497734 A1 and as outlined in Scheme 5. In this process benzamidine hydrochloride is first allow to react with a chloroformate and the resulting product is then dimerized. The resulting 2-hydroxy-4,6-bisaryl-1,3,5-triazine is converted to 2-chloro-4,6-bisaryl-1,3,5-triazine by treatment with thionyl chloride, which is subsequently allowed to react with resorcinol to synthesize 2-(2,4-dihydroxyphenyl)-4,6-bisaryl-1,3,5-triazine, as shown in Scheme 5. 
An alternate approach for the preparation of 2-chloro-4,6-bisaryl-1,3,5-triazines is based on the reaction of aryl nitrites with phosgene in the presence of HCl in a sealed tube (S. Yanagida, H. Hayama, M. Yokoe, and S. Komori, J. Org. Chem., 1969, 34, 4125. Another approach is the reaction of N,N-dimethylbenzamide with phosphoryl chloride complex which is then allowed to react with N-cyanobenzamidine to form 2-chloro-4,6-bisaryl-1,3,5-triazine (R. L. N. Harris, Synthesis, 1990, 841). Yet another approach involves the reaction of polychloroazalkenes, obtained from the high temperature of chlorination of amines, with amidines to form 2-chloro-4,6-bisaryl-1,3,5-triazines (H. G. Schmelzer, E. Degener and H. Holtschmidt, Angew. Chem. Internat. Ed., 1966, 5, 960; DE 1178437). None of these approaches are economically attractive, and thus are not commercially feasible.
Finally, there are at least three approaches which do not require the intermediacy of 2-chloro-4,6-bisaryl-1,3,5-triazine for the preparation of the parent compound, 2-(2,4-dihydroxyaryl)-4,6-bisaryl-1,3,5-triazine. These approaches utilize benzonitriles or benzamidines as starting materials (See U.S. Pat. Nos. 5,705,643 and 5,478,935; WO 96/28431). The benzamidines are condensed with 2,4-dihydroxybenzaldehyde followed by aromatization (Scheme 6) or condensed with phenyl/alkyl 2,4-dihydroxybenzoates (Scheme 7) or 2-aryl-1,3-benzoxazine-4-ones (Scheme 8) to form 2-(2,4,-dihydroxyaryl)-4,6-bisaryl-1,3,5-triazine. These approaches have the drawback that the starting materials are expensive and may require additional steps to prepare. Moreover, overall yields are not satisfactory and the processes are not economically attractive.
Based on Benzamidine reactions with 2,4-dihydroxybenzaldehyde: 
In summary, although direct Lewis acid catalyzed bisarylation of cyanuric chloride to form the desired 2-chloro-4,6-bisaryl-1,3,5-triazine intermediate is the most economically attractive approach, this process has found only limited use due to the following problems:    1. Poor selectivity: Almost total lack of selectivity for bisarylation (with the exception of m-xylene where some selectivity is observed). Mono- and tris-arylated triazines are the major by-products.    2. Poor reactivity: Typical reaction conditions require high temperatures, long reaction times, and variable temperatures during the course of reaction. Aromatics with electron-withdrawing groups (such as chlorobenzene) fail to react beyond mono-substitution even at elevated temperatures and long reaction times.    3. Safety hazards: Temperature and addition rate must be carefully monitored to avoid an uncontrollable exotherm which may result in safety hazards.    4. Poor process conditions: The reaction slurry is either thick and difficult to stir or solid thereby making stirring impossible. The process requires various reaction temperatures and addition of reactants in portions over several hours.    5. Isolation problem/poor isolated yield: Separation and purification of the desired product is difficult and isolated yields are generally poor and commercially unacceptable.    6. Not a general process: The reaction cannot be used with different aromatics other than m-xylene.
Thus, there remains a need for improved methods for synthesizing triazine UV absorbers.